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The flow behind the variable area nozzle for radial turbines was measured with a 3-hole yaw probe and calculated with CFD. Two nozzle throat-areas were investigated: the smallest and the largest opening for the variable nozzle. Test results agreed with the calculated results qualitatively. The leakage flow through the tip clearance of the nozzle vane significantly affected the flow field downstream of the nozzle vane with the smallest opening. However, the effect on leakage flow on the flow field downstream of the nozzle vane with the largest opening was very weak. In the flow field of the largest opening nozzle, the effect of wake s dominant. The effect of the clearance of the nozzle vane on the turbine performance was estimated by a 1D-model and the strong influence on the turbine efficiency was confirmed at smallest opening. The flow fields in the impeller downstream of the nozzle vane at the smallest opening with and without the nozzle clearance were investigated with CFD. The setting angle of the nozzle vane without clearance was adjusted to match the operating point of the turbine with the nozzle clearance. In order to extract the specific work from the impeller, the nozzle vane with the vane clearance requires the larger vane setting angle than that without clearance. The increase of the vane setting angle increases the incidence loss and deteriorates turbine efficiency.

This paper describes the ways to increase pressure ratio of centrifugal compressors on condition of fixed impeller diameter. The compressors discussed in the paper were designed for turbochargers applied to 500kW class Diesel engines and their impeller diameters were about 140mm. Pressure ratio was increased from 3.6 to 4.8 at first and then increased from 4.8 to 5.7. The pressure was raised by means of increase in rotational speed together with additional increase in pressure coefficient. The pressure coefficient was increased by means of increase in work coefficient. The increases in rotational speed and work coefficient lead to increase in relative Mach number at inducer. However, the relative Mach number at inducer of the compressor with a pressure ratio of 3.6 already exceeded 1.3. Therefore, it was expected that complicated flow features related to the occurrence of shock wave would deteriorate the new compressors’ efficiency and operating range. In order to avoid this problem, following new measures were taken with conventional design methods. (1) Reduction of a blade loading — an increase in number of blade by means of double splitter (two adjoining splitters). (2) Introduction of technologies to enhance an operating range — recirculation device. (3) Selection of a proper diffuser to convert kinetic energy at an impeller exit to static pressure — vaned diffuser with elliptic leading edge. Newly designed compressors realized expected performance, stage pressure ratio and efficiency. Suitability of these measures was confirmed.

The operating points of a turbocharger compressor tend to approach or cross its surge line while an engine is accelerating, particularly under low-engine speed conditions, hence the need for an acceptable surge margin under low compressor-speed conditions. A method shifting the stability limit on a compressor low-speed line toward a lower flow rate is expected and inlet recirculation is often observed in a centrifugal compressor with a vaneless diffuser near a surge and under a low compressor-speed condition. First, examples of inlet recirculation were introduced in this paper, whereupon the effect of inlet recirculation on compressor characteristic was discussed by 1-D consideration and the potential shown for growth of inlet recirculation to destabilize compressor operations. Accordingly, this study focused on suppressing the effect of inlet recirculation on compressor characteristics using small fins mounted in a compressor-inlet pipe, and examining whether they enhance the compressor operating range under low-speed conditions. Small fins are known as inlet fins in this paper. According to test results, they showed great promise in enhancing the compressor operating range during inlet recirculation. Besides, attempts were also made to investigate the qualitative effect of inlet fins on flow fields using CFD and the disadvantages of inlet fins were also discussed.

The flow behind the variable area nozzle which corresponds to the flow at the leading edge of the impeller was measured with a 3-hole yaw probe and calculated with CFD. Two nozzle throat-areas were investigated. One is the smallest and the other is the largest opening for the variable nozzle. Test results agreed with the calculated results qualitatively. The leakage flow through the tip clearance of the nozzle vane significantly affected the flow field downstream of the nozzle vane with the smallest opening. However, the effect on leakage flow on the flow field downstream of the nozzle vane with the largest opening was very weak and the effect of wake is dominant.